Spectral reflectometry is based on acquiring the reflected spectrum from the sample of interest, normalizing it relative to the reflected spectrum of a known reference sample, and then fitting a model to the normalized spectrum. The values of film parameters (thickness of layers, refractive index, etc.) that produce the best fit between the theoretical model and the measured spectrum are reported as the results.
Standard spectral-reflectometry systems and similar optical metrology methods are designed for samples that include films stacked on a polished substrate. Conventionally, the acquired data is normalized using data for a polished reference sample and a model is fit to the normalized spectrum. Where the sample includes a textured substrate, such as used in photovoltaics, a complex theoretical model is required to characterize the effects of the substrate texture on the sample reflectivity. Additionally, the illumination conditions that are used to obtain the data for the polished reference sample are very different from the illumination conditions to measure a textured sample, which further increases the complexity of the metrology.
In high volume solar cell manufacturing, where highly textured substrates are commonly used, the common practice in production monitoring is to periodically insert a “witness” sample, which includes a polished substrate. The witness sample is processed along with the production samples and parameters of the witness sample, such as film thickness, is measured using standard reflectometry, assuming that the measured parameter on the witness sample is correlated to the same parameter on the textured production sample. While standard reflectometry is easier to perform on an untextured witness sample than preparing complex models to characterize the effects of texturing on the sample reflectivity, the correlation to the textured production sample may be poor. In addition, the witness samples introduce an added cost to production for facilitating process monitoring.
Accordingly, improvements for optical metrology of textured samples are desirable.